The present invention provides an economical, rapid and efficient method for making metal and metal oxide powders.
Metals and metal oxide powders enjoy a multitude of applications. For example, metals and metal oxide powders are suitable for use in the application fields of powder metallurgy, catalysts, hardmetals, electrochemical devices (including batteries, capacitors, photovoltaics, sensors and fuel cells), metal matrix composites, chemicals (such as electroplating and as raw materials for metal-organic compositions), magnetic compositions, polymer fillers, pigments, optical absorbers, metal injection-molding, electrical and magnetic shielding, display materials, precursors for thin and thick film applications, thermal spray, electronics (including conductors and dielectrics), ceramics, integrated circuits, and brazing alloys, among others.
Metals and metal oxide powders commonly are manufactured by decomposing, oxidizing or reducing a metal carbonate, hydroxide or oxide, or other metal-containing compound. The basic mechanisms and kinetics of these reactions generally are well established. Conventional conversion of metal-containing compounds to metals and metal-oxides typically are carried out in pusher, strip-belt, rotary, or fluidized bed reactors. To achieve complete conversion, the reactants typically require a residence time on the order of hours.
For example, the conversion of cobalt hydroxide by decomposition and hydrogen reduction to form cobalt conventionally requires a residence time of from one to two hours at a temperature of 500 to 800xc2x0 C. in a strip belt or pusher reactor. The conversion of cupric hydroxide to cupric oxide by decomposition and oxidation in a strip-belt or pusher reactor requires a residence time of from one to three hours at a temperature of 150 to 800xc2x0 C.
A variety of alternative methods of forming metals and metal oxides from metal-containing compounds have been proposed. For example, the formation of metallic powders by plasma vaporization of inorganic compounds is disclosed in U.S. Pat. Nos. 5,788,738, and 5,851,507.
Though plasma vaporization and similar methods are scientifically interesting, the expense of required equipment and the low production rates of these methods make them unsuitable for large-scale commercial application.
It has been taught that carbothermal reduction of metal-containing compounds to form metallic carbides and nitrides may be conducted at rapid heating rates. For example, U.S. Pat. No. 5,194,234 describes a carbothermal reduction method of forming fine powdered boron carbide by reacting a mixture of boric oxide or hydrate and a carbon source at a temperature above about 1400xc2x0 C. and cooling the resultant product. In the method it is preferred to heat the reaction mixture at a rate equal to or exceeding 1000xc2x0 C./second.
U.S. Pat. Nos. 5,190,737 and 5,340,417 disclose methods of preparing silicon carbide by carbothermal reduction involving heating a mixture of a silica source and a carbon source at a heating rate at least about 100xc2x0 C./second. U.S. Pat. Nos. 5,380,688 and 5,746,803 disclose methods employing rapid carbothermal reduction which involve heating reactants at rates from 100xc2x0 C. to 100,000,000xc2x0 C./second to from metallic carbides. And U.S. Pat. No. 5,756,410 discloses a method of forming metal carbonitrides which method includes heating reactants at rates from 100xc2x0 C. to 100,000,000xc2x0 C./second.
However, the effective use of rapid heating rates and short residence times in the conversion of metal-containing compounds into metals and metal oxides has not been shown.
This invention is a method for converting a metal-containing compound into the metal or an oxide of the metal of the metal-containing compound, which comprises heating the metal-containing compound at a rate between about 100xc2x0 C./second to about 100,000,000xc2x0 C./second to an elevated temperature that makes conversion of the metal-containing compound thermodynamically favorable, and holding the metal-containing compound at the elevated temperature for a residence time sufficient to substantially convert the metal-containing compound into at least one product selected from the group consisting of (i) the metal and (ii) oxides of the metal.
The present invention is based on the discovery that the kinetics of conventional methods of converting metal-containing compounds to metals and metal-oxidesxe2x80x94such as decomposition, oxidation and reductionxe2x80x94are much faster than previously known. By rapidly heating a metal-containing compound, it has been found, the compound may be converted to metal or metal-oxide in seconds or fractions of seconds. The lengthy, hours-long reactor residence times of convention have been overcome by the present invention.
Thus, a method of producing metals and metal oxides efficiently at a high production rate at relatively low cost is provided by the present invention.
Typical reactions useful in the practice of this invention for converting a metal-containing compound into the metal or an oxide of the metal of the metal-containing compound include decomposition, oxidation, reduction, substantially simultaneous decomposition and reduction, and substantially simultaneous decomposition and oxidation. Accordingly, embodiments of this invention include the methods for (1) decomposing a metal-containing compound to produce the metal or an oxide of the metal of the metal-containing compound, (2) reducing a metal-containing compound to produce the metal or an oxide of the metal of the metal-containing compound, (3) oxidizing a metal-containing compound to produce the metal or an oxide of the metal of the metal-containing compound, (4) substantially simultaneously decomposing and reducing a metal-containing compound to produce the metal or an oxide of the metal of the metal-containing compound, and (5) substantially simultaneous decomposing and oxidizing a metal-containing compound to produce the metal or an oxide of the metal of the metal-containing compound; all of which methods comprise heating the metal-containing compound at a rate of between about 100xc2x0 C./second to about 100,000,000xc2x0 C./second to an elevated temperature that makes conversion of the metal-containing compound thermodynamically favorable and holding the metal-containing compound at the elevated temperature for a residence time sufficient to substantially convert the metal-containing compound into at least one product selected from the group consisting of (i) the metal and (ii) oxides of the metal.
In practicing the present invention, some metal and metal oxides may be produced by a two-step process. In step one, the metal-containing compound undergoes conversion to a xe2x80x9cprecursorxe2x80x9d metal-containing compound in which conversion is not substantially complete. In step two (the finishing step), the product from step 1 is heated for a second time at a temperature sufficient to form the final metal or metal oxide product. This two-step process is not needed in all cases, but may be used when necessary or desired. The second heat treatment may be carried out using the technology of the present invention or may be carried out using conventional methods.
In addition, the present invention may be practiced by first converting a metal-containing compound to the metal (xe2x80x9cprecursorxe2x80x9d) of the metal-containing compound, then in a second step, converting the metal to an oxide of the metal. Also, the present invention may be practiced by first converting a metal-containing compound to a first (precursor) oxide of the metal of the metal-containing compound, then in a second step converting the precursor metal oxide into the metal or a second, different oxide of the metal of the metal-containing compound.